Forty-four years ago, in between A Clockwork Orange and the Beatles' sixth LP, Anthony Burgess published Here Comes Everybody, a critical study of James Joyce intended for readers who had been "scared off by the professors". Joyce, difficult? Not at all, Burgess said: "If ever there was a writer for the people, Joyce was that writer." Burgess polished off his book in eight months; Declan Kiberd has spent three decades working towards his. But his title is similarly inclusive and he, too, wants to demolish "the legend of forbidding difficulty" that has "scared readers off". On the cover is an Eve Arnold photo showing how it should be: a young Marilyn Monroe devouring the final pages of Ulysses

Kiberd tells the story of his father, a Dubliner who loved Ulysses and knew it by heart, but who, having been enticed to attend a Joyce symposium at Trinity College, bolted for the door almost as soon as he'd arrived. Though himself an academic, Kiberd is dismayed that a book which set out to celebrate the common man and woman isn't read by them - or, indeed, by "most students, lecturers and intellectuals", only by paid-up Joyceans. Hemingway professed to admire Joyce, yet all but a few pages of his copy of Ulysses remained uncut. More recently Roddy Doyle set the cat among the pigeons when he complained that the novel had been overpraised and "could have done with a good editor".

Kiberd concedes Doyle's point: the notion of Ulysses's "monumental perfection" is silly, he says. But he rebuts the charge that the novel is inaccessible. Joyce wasn't especially erudite, he argues. Unlike his snooty modernist peers, he was a socialist and democrat who believed in mass literacy - and was happier discussing Dickens with post office workers than he was sitting in bohemian cafes. Reading Ulysses may be a challenge, but so are most jobs. We shouldn't need a sacred priesthood to interpret it for us.

"Oh rocks, tell us in plain words," Molly Bloom says, and Kiberd heeds the message: despite references to Adorno, Auerbach, Benjamin and the like, he avoids literary jargon and theory. Most of the book consists of critical commentary, old-fashioned in its textual attentiveness, bang up to date in its emphasis on Joyce as anti-racist, anti-colonialist, pro-women, pro-recycling and anti-war. Patiently guiding us through the 18 chapters of Ulysses, Kiberd doesn't claim to offer new findings - after almost a century of Joycean scholarship, that would hardly be possible. But his interpretation of the novel as humane and populist is refreshing. Here's a book in which even the humblest and most marginal characters are granted an interior monologue. And self-absorbed though they are, the monologues have a commonality: by accessing private thought processes, they put us in touch with each other.

Kiberd covers a lot of ground, some of it familiar (time, dreams, circularity, stream of consciousness), some more surprising (waste, deforestation and Ulysses as a "counter-newspaper"). Two themes stand out. The first is his reading of Bloom as pan-sexual or androgynous - more the dainty househusband than l'homme moyen sensuel. Bloom's kindness and equanimity offer a new model of heroism. His shepherding of a drunk Stephen Dedalus is part of this. So is his tolerance of Molly's affair with Blazes Boylan. While others are overbearing, Bloom's forbearance wins the day.

The other (connected) thesis is that Ulysses is a novel less about June 1904 (when it is set) than about 1914-21 (when it was being written). The context of Joyce's "extended hymn to the dignity of everyday living" was the first world war, Kiberd argues: rather than heroic narratives and blood sacrifice, Joyce gives us ordinary Dublin folk going about their daily business. British oppression and the great famine lurk in the background, but so do the Easter rising and home rule. And at the heart of it there's Bloom, a domesticated, part-assimilated Jew who challenges the jingoistic prejudices of the era.

There are times when Kiberd forgets himself and insists that Ulysses is perfect after all. If we feel lost reading it, that's "exactly how Joyce wants readers to feel". If pastiche takes over, that's because style is the subject matter of the book. If the language is sometimes redundant and over-learned, that's Joyce's protest "against the world which is taking cultural value away". There's no doubting Kiberd's enthusiasm, but he doesn't always convince: can it really be true, for instance, that Molly Bloom is "utterly and solely present in her own words, like no previous character" and that she becomes "the voice of the book as a whole"? (More sceptical readers may sympathise with Nora Joyce, who said that "Jim knows nothing at all about women".) Kiberd might have made his task simpler by admitting that Ulysses is structurally flawed; that the 200-plus pages given to the Circe and Oxen of the Sun chapters wrench it out of shape; that it's a book to be kept by the bed, like a collection of poems, rather than read for its narrative.

Flawed or not, Ulysses has a vital role as a "wisdom narrative" or moral primer. As Kiberd says, it gives us "advice on how to cope with grief; how to be frank about death in the age of its denial; . . . how to walk and think at the same time". It's how they walk and think that initially sets Bloom and Stephen apart: one is practical-minded and acutely observant, the other over-educated and myopic. But both are grieving (one for his mother, the other for his son), and their convergence in the small hours is the novel's key moment - a coming together as triumphant as Molly's final orgasmic "yes". Did Marilyn Monroe, reading those last pages, realise that Molly is masturbating? Is that why she's so enthralled? She certainly doesn't look shocked. Nor should she be. As Joyce said, "If Ulysses isn't fit to read, life isn't fit to live."